JavaScript Variables

If you translate variables directly into Burmese, it means "changing values." To visualize it, you can think of a variable as a specific location in computer memory where you can store any value you like. JavaScript is case-sensitive and uses the Unicode character set. For example, you can even use the German word “Früh” as a variable name.

const Früh = "foobar";

The variable Früh is not the same as the variable früh because JavaScript is case-sensitive.

In the past, variables were declared using the var keyword. For example:

var mySkillIssues = 42;

The commonly used variable types in JavaScript are:

• numbers: Represents integers and decimal floating-point numbers.
• boolean: true or false.
• string: Refers to text/strings.
• undefined: Refers to a variable that has not been assigned a value yet.

var smsSendingLimit = 100; 
var isAdmin = true; 
var username = "wagslane"; 
var nothing = undefined;

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let and const

In modern JavaScript today, the var keyword is rarely used; instead, let and const are preferred. You can reassign a new value to variables declared with let. For example:

let username = "dengar_the_bh"; 
username = "boba_fett";

Variables declared with const are constant values, meaning you cannot reassign a new value to them. In JavaScript, you must assign a value immediately when declaring a const variable. Unlike let, you cannot leave it unassigned.

let myVar; // OK 
const MYVAR; // SyntaxError

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Variable Scope

In JavaScript, variable scope determines the visibility and accessibility of variables. Visibility and Accessibility mean defining which part of the program can see and use which variable.

Global Scope

Variables declared outside of functions or code blocks { } are global scope variables. Global scope variables can be accessed and modified from anywhere in the program.

let globalVar = "I am global";  

function test() {  
    console.log(globalVar); // Accessible inside functions 
}

Function Scope

Variables declared inside a function have function scope. All three types—let, var, and const—can be used within a function scope. A variable inside a function scope cannot be accessed from the outside global scope or block scopes. In fact, those scopes won't even know the function-scoped variable exists.

function greet() {  
    var localVar = "Hello";  
    console.log(localVar); // Works perfectly 
}  

greet(); 
console.log(localVar); // Throws ReferenceError

Block Scope

Variables declared inside { } are called block scope variables. These variables can only be used within those specific { } brackets and cannot be accessed from an outside global or function scope. Only let and const respect block scope; var does not follow block scope rules. A variable declared with var inside { } can still be accessed from its closest surrounding function scope or global scope.

if (true) {  
    let blockLet = "Secure";  
    var leakedVar = "Unsecured"; 
}  

console.log(leakedVar); // Logs "Unsecured" 
console.log(blockLet); // Throws ReferenceError

References

JavaScript - Quick & Concise (Ei Maung - Fairway)

MDN Web Docs - Introduction to JavaScript

Boot.dev